1. Field of the Invention
This invention generally relates to filters and duplexers, and more particularly, to a filter having upper and lower electrodes provided on opposite surfaces of a piezoelectric film in which an overlapping region of the upper and lower electrodes across the piezoelectric film has an elliptical shape and a duplexer with such a filter.
2. Description of the Related Art
Recently, there has been an increasing demand for filters with compact and light resonators and filters configured by combining these filters as wireless communication equipment that may be typically cellular phones has spread rapidly. In the past, dielectric filters or surface acoustic wave (SAW) filters were mainly used. Nowadays, piezoelectric thin film resonators and filters using these resonators are attracting attention, in which the piezoelectric thin film resonators are characterized in that they gave good high-frequency performance and are compact and producible as a monolithic device.
The piezoelectric thin-film resonators may be categorized into an FBAR (Film Bulk Acoustic Resonator) type and an SMR (Solidly Mounted Resonator) type. The FBAR has a primary structure composed of an upper electrode, a piezoelectric film and a lower electrode, and a gap provided below the lower electrode and located within an overlapping region (resonance portion) in which the upper and lower electrodes overlap with each other across the piezoelectric film. The gap may be defined between the lower electrode and a silicon substrate by wet-etching a sacrificed layer on a main surface of the silicon substrate. The gap may also be formed by wet- or dry-etching the substrate from the backside thereof. The SMR employs an acoustic reflection film instead of the gap, in which a first film having a relatively high acoustic impedance and a second film having a relative low acoustic impedance are alternately laminated with a film thickness of λ/4 where λ is the wavelength of an acoustic wave of the resonator.
The upper and lower electrodes may be made of aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir) or the like. The piezoelectric thin film may be made of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconium titanate (PZT), lead titanate (PbTiO3) or the like. The substrate may be made of glass other than silicon.
In the above-mentioned piezoelectric thin film resonator, an unnecessary wave of a lateral mode is excited in addition to the thickness longitudinal vibration wave. The wave of the lateral mode is propagated in parallel with the electrode surfaces and is reflected by ends of the electrodes or gap. The wave of the lateral mode may cause an unnecessary spurious component in the impedance characteristic of the resonator and a ripple in the pass band of the filter. The problems result in some difficulties in applications.
Japanese Patent Application Publication No. 2005-124107 discloses an improved a piezoelectric thin-film resonator and a filter using the same directed to restraining influence of the lateral wave in which the overlapping portions of the lower and upper electrodes have an elliptical shape having a major-axis length a and a minor-axis length designed so as to satisfy 1.0<a/b<1.9.
The elliptical shape of the overlapping portions of the upper and lower electrodes reduces the spurious components in the resonance performance of the resonator, as compared to a comparative filter having a circular shape of the overlapping portions. However, it may be desired to further improve the resonance performance and reduce the ripple in the pass band and to further improve the filter insertion loss and group delay characteristic.